PERF: Admin plugin preload settings routes (#31215)

Followup 503f9b6f02ac5c4918d41611848c886b8755e5a0

This previous commit introduced an autogenerated
settings route for every plugin with more than one
setting defined. Plugins with only one setting
only have enabled_site_settings defined, which are
handled using the toggle in the admin plugin list,
so we don't need a dedicated setting page for them.

However in production this introduced a performance
issue, since we were looking through SiteSetting.all_settings
for every plugin, which could be quite slow in some
cases especially on our hosting.

Instead, we already have all the plugin settings cached
inside `SiteSetting.plugins`. We can instead use this to
count how many settings the plugin has, then if there is > 1
for a plugin we use the settings route. This is a much faster lookup
than
searching through SiteSetting.all_settings.

spec/lib/plugin/instance_spec.rb

@@ -1103,4 +1103,80 @@ TEXT
       )
     end
   end
+
+  describe "#full_admin_route" do
+    context "when there is no admin route defined for the plugin" do
+      context "if the plugin has more than one setting" do
+        before do
+          plugin_instance.stubs(:plugin_settings).returns(
+            { enabled_setting: plugin_instance.name, other_setting: plugin_instance.name },
+          )
+        end
+
+        it "returns the default settings route" do
+          expect(plugin_instance.full_admin_route).to eq(
+            {
+              auto_generated: true,
+              full_location: "adminPlugins.show",
+              label: "discourse_sample_plugin.title",
+              location: "discourse-sample-plugin",
+              use_new_show_route: true,
+            },
+          )
+        end
+      end
+
+      context "if the plugin has only one setting (which is the enabled setting)" do
+        before do
+          plugin_instance.stubs(:plugin_settings).returns({ enabled_setting: plugin_instance.name })
+        end
+
+        it "returns nothing" do
+          expect(plugin_instance.full_admin_route).to be_nil
+        end
+      end
+
+      context "if the plugin is not configurable" do
+        before { plugin_instance.stubs(:configurable?).returns(false) }
+
+        it "returns nothing" do
+          expect(plugin_instance.full_admin_route).to be_nil
+        end
+      end
+    end
+
+    context "when there is an admin route defined for the plugin" do
+      context "when using the new show route" do
+        before { plugin_instance.add_admin_route("test", "testIndex", use_new_show_route: true) }
+
+        it "returns the correct details" do
+          expect(plugin_instance.full_admin_route).to eq(
+            {
+              auto_generated: false,
+              full_location: "adminPlugins.show",
+              label: "test",
+              location: "testIndex",
+              use_new_show_route: true,
+            },
+          )
+        end
+      end
+
+      context "when not using the new show route" do
+        before { plugin_instance.add_admin_route("test", "testIndex") }
+
+        it "returns the correct details" do
+          expect(plugin_instance.full_admin_route).to eq(
+            {
+              auto_generated: false,
+              full_location: "adminPlugins.testIndex",
+              label: "test",
+              location: "testIndex",
+              use_new_show_route: false,
+            },
+          )
+        end
+      end
+    end
+  end
 end